1. Field of the Invention
The present invention relates to an optical pickup apparatus usable in an optical disk drive.
2. Description of the Related Art
Japanese Laid-Open Publication No. 9-312033 discloses an integration type optical pickup apparatus 500 shown in FIG. 5.
The optical pickup apparatus 500 includes a semiconductor laser 101 as a light source for emitting laser light, a polarization beam splitter 102 for splitting the light emitted by the semiconductor laser 101, a mirror 103 for reflecting the light reflected by an optical disk (not shown) after being emitted by the semiconductor laser 101, a ¼ wave plate 104 for converting linearly polarized light from the semiconductor laser 101 into circularly polarized light, a light detector 106 for detecting the light reflected by the optical disk, and a diffraction device 105 including two areas for directing the light emitted by the semiconductor laser 101 and reflected by the optical disk toward the light detector 106.
FIG. 6 is a schematic plan view of the diffraction device 105. As shown in FIG. 6, the diffraction device 105 includes two areas G and H.
FIG. 7 is a schematic plan view of the light detector 106. As shown in FIG. 7, the light detector 106 includes a first light receiver 106a and a second light receiver 106b which are both provided on one stem 500b (FIG. 5) while being at a distance from one another. A light receiving plane of the first light receiver 106a is equally divided along two dividing lines into four quadrangular light receiving regions A, B, C and D. The first light receiver 106a is provided so that a line vertical to the plane thereof matches the optical axis of the light incident on the diffraction device 105, and so that the two dividing lines are respectively parallel and perpendicular to a tracking direction of the optical disk. A light receiving plane of the second light receiver 106b is equally divided into two light receiving regions E and F along a dividing line 106c. 
The above-described elements of the optical pickup apparatus 500 are integrally structured. Returning to FIG. 5, the semiconductor laser 101 and the light detector 106 are accommodated in a housing 500a. On the housing 500a, the polarization beam splitter 102 and the mirror 103 are provided. The polarization beam splitter 102 is provided so that a center line thereof matches the optical axis of the semiconductor laser 101. The mirror 103 is provided parallel to the plans of polarization of the polarization beam splitter 102 and is oriented so that light reflected by the mirror 103 is vertically incident on the light detector 106. On a light outgoing surface of the polarization beam splitter 102, the ¼ wave plate 104 is provided so that an optic axis thereof inclines at 45 degrees with respect to the polarization direction of the light from the semiconductor laser 101. The diffraction device 105 is provided on a surface of the polarization beam splitter 102, the surface facing the mirror 103.
The optical pickup apparatus 500 operates as follows. Light emitted by the semiconductor laser 101 is transmitted through the polarization beam splitter 102. The light, which is linearly polarized, is converted into circularly polarized light by the ¼ wave plate 104 before leaving the optical pickup apparatus 500. Then, the light is transmitted through an optical system (not shown) including an objective lens and a collimator lens, and is focused onto the optical disk as a recording medium.
The light reflected by the optical disk is incident again on the optical pickup apparatus 500 through the ¼ wave plate 104. The light which has been transmitted through the ¼ wave plate 104 is linearly polarized in a direction which is perpendicular to the polarization direction of the linearly polarized light emitted by the semiconductor laser 101. The light is then reflected by the polarization beam splitter 102 and is incident on the diffraction device 105.
Of the light which is transmitted through the diffraction device 105, a light component which has not been diffracted (zero'th order diffracted light component) is reflected by the mirror 103 and reaches the first light receiver 106a (FIG. 7). Whereas, a light component diffracted by the area G or H of the diffraction device 105 (FIG. 6) (first order diffracted light component) is focused on the central dividing line 106c of the second light receiver 106b (FIG. 7). When the optical disk moves away from the optical pickup apparatus 500, the focal point of the first order diffracted light component moves toward the light receiving region E or F from the central dividing line 106c of the second light receiver 106b. When the optical disk moves closer to the optical pickup apparatus 500, the focal point of the first order diffracted light component moves in the opposite direction (i.e., toward the light receiving region F or E from the central dividing line 106c).
An RF signal as an information reproduction signal is detected from a sum of signals which are output from the light receiving regions A, B, C and D of the first light receiver 106a. A tracking error signal is detected from signals each indicating a sum of signals output from the light receiving regions located diagonally (A+C, B+D) using a phase contrast method. A focusing error signal is detected from a signal indicating a difference between signals output from the light receiving regions (E–F) using the Foucault method.
When the temperature of the optical pickup apparatus 500 (FIG. 5) changes, the oscillation wavelength of the light from the semiconductor laser 101 changes, which in turn changes a diffraction angle of the first order diffracted light component. Accordingly, the focal point is shifted in a direction substantially perpendicular to the central dividing line 106c (FIG. 7) of the second light receiver 106b. As a result, the focusing error signal is slightly offset. This offset is generally known to be suppressed by inclining the central dividing line 106c. 
However, when the temperature of the optical pickup apparatus 500 (FIG. 5) changes, the stem 500b and the optical components included in the optical pickup apparatus 500 expand or contract. This may also offset the focusing error signal in the case where the light detector 106 is located in a certain manner with respect to the other components.
In the case of the optical pickup apparatus 500 described in Japanese Laid-Open Publication No. 9-312033, the semiconductor laser 101 and the light detector 106 are provided on the same stem (or base) 500b. The central dividing line 106c (FIG. 7) is substantially perpendicular to a phantom straight line connecting the light emitting point of the semiconductor laser 101 and the focal point of the light on the light detector 106. Since the distance between the semiconductor laser 101 and the light detector 106 changes due to expansion or contraction of the stem 500b when the temperature of the optical pickup apparatus 500 changes, the first order diffracted light component from the diffraction device 105 is shifted toward the light receiving region E or F from the central dividing line 106c of the second light receiver 106b. Thus, the focusing error signal is offset. Since the polarization beam splitter 102 also expands or contracts in accordance with the temperature change, the relative positions of the polarization beam splitter 102 and the mirror 103 change, which also offsets the focusing error signal.
The conventional optical pickup apparatus can suppress the offset of the focusing error signal generated by the change in the wavelength, by inclining the central dividing line for dividing the light detector into two. However, the focusing error signal is also offset by the expansion or contraction of the optical components and the stem. Accordingly, the conventional optical pickup apparatus has a problem in that it is difficult to accurately record and/or reproduce information when the temperature thereof is changed.